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The dilemma
You’re sat in your car on a scorching summer day, wondering what’s the most economical way to cool down.
You have two choices: turn on the air conditioning to circulate cold air around the cabin or open the windows to feel a refreshing breeze.
Unfortunately, both options harm your fuel economy. The AC uses energy from your engine (or battery if you drive an EV), while winding the windows down increases drag making your car work harder to maintain the same speeds.
So, which choice is best for your car and your wallet?
What the science says
Air con vs windows has been debated for many years, with numerous studies attempting to settle the issue. Even the Discovery Channel’s MythBusters have had a go at answering the age-old question.
The Society of Automotive Engineers (SAE) has investigated this topic twice with the most recent 2013 study concluding that winding your windows down is the best for your fuel economy (a conclusion MythBusters also reached).
So, it’s a win for windows down? Not exactly. The SAE’s study isn’t entirely conclusive because there’s a range of variables not accounted for such as how open the windows are, whether the air con is on full blast, the speed the car is travelling, and the type of car being driven.
AC systems can increase fuel consumption by as much as 10% in newer cars, with older models being even less efficient.
Meanwhile, drag affects aerodynamic cars more significantly. The SAE's 2004 study showed that driving at 50 mph with the windows down reduced a saloon’s efficiency by 20%, while an SUV's efficiency dropped by just 8%.
This means that even though rolling your windows down is often the more fuel-efficient choice, what’s best for you and your car depends on various factors.
What’s best?
As a general rule of thumb, opening the windows is better for your fuel economy when driving at slower speeds around town or when stuck in bumper-to-bumper traffic.
This is because drag has less impact at lower speeds, whereas your AC’s energy consumption remains more or less constant.
However, if you’re driving at motorway speeds, you’re probably better off keeping the windows up and putting the air con on – especially if you’re driving a modern and streamlined car.
For most driving styles though, a small crack in your window is likely best for your MPG. The choice is yours!
How else can I keep my car cool?
Here are some more tips on how to keep your car cool this summer:
- Park in the shade or garage
- Leave a 1cm gap in your window when parked
- Use a heat deflector
- Use a solar-powered fan
- Open the car doors/windows to let hot air escape before setting off
- Stay hydrated with water onboard
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